The present invention relates to a non-aqueous electrochemical apparatus using a non-aqueous electrolyte. More particularly, it relates to a non-aqueous electrolyte lithium secondary battery.
Non-aqueous electrochemical apparatuses using light metals such as lithium and sodium as negative electrode active materials are used in a wide variety of the fields such as of various electric and electronic equipment. Non-aqueous electrochemical apparatuses include batteries, capacitors for electric double layers, electrolytic capacitors and the like. Especially, non-aqueous electrolyte secondary batteries are being intensively investigated and developed at present because they are rechargeable batteries which have high energy density and can be miniaturized and weight-saved.
Non-aqueous electrolyte secondary batteries are composed of positive electrodes, negative electrodes, a non-aqueous electrolyte, and separators separating the positive electrode and the negative electrode from each other.
As the positive electrode active materials, there are used lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMn2O4, LiMnO2) and lithium ferrate (LiFeO2), and these compounds in which a part of their transition metals (Co, Ni, Mn, Fe) is replaced with other transition metals, tin (Sn), aluminum (Al), and the like; transition metal oxides such as vanadium oxide (V2O5), manganese dioxide (MnO2) and molybdenum oxide (MoO2, MoO3); transition metal sulfides such as titanium sulfide (TiS2), molybdenum sulfide (MOS2, MoS3) and iron sulfide (FeS2); and the like. In preparing positive electrode using these positive electrode materials, carbon black is added as a conducting agent for compensation of low electronic conductivity of the positive electrode active materials and, simultaneously, for enhancement of electrolyte retainability in the electrode plates.
As the negative electrode active materials, lithium ion or sodium ion is used, and as negative electrode host materials, there are used graphite materials such as amorphous carbon materials, artificial graphite fired at a temperature of 2000° C. or higher, and natural graphite; alkali metals or alloys of alkali metals with aluminum (Al), lead (Pb), tin (Sn), bismuth (Bi), silicon (Si), and the like; cubic system intermetallic compounds which can intercalate alkali metal between their crystal lattices (AlSb, Mg2Si, NiSi2); lithium nitrogen compounds (Li(3-x)MxN (M=transition metal)); and the like.
Recently, non-aqueous electrolyte secondary batteries using for negative electrodes the above host materials capable of absorbing and releasing alkali metal ions have a greater part of these kinds of batteries.
As the solvents for the electrolytes, there are mainly used cyclic carbonate esters such as propylene carbonate (PC) and ethylene carbonate (EC), chain carbonate esters such as diethyl carbonate (DEC) and dimethyl carbonate (DMC), cyclic carboxylate esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL), chain ethers such as dimethoxymethane (DMM) and 1,3-dimethoxypropane (DMP), cyclic esters such as tetrahydrofuran (THF) and 1,3-dioxolan (DOL), and the like.
In using these solvents for non-aqueous electrolyte secondary batteries, they are preferably high in electrical conductivity. For this purpose, solvents high in specific permittivity and low in viscosity are preferred. However, to have a high specific permittivity means to have a strong polarity and a high viscosity. Therefore, in the electrolytes of the present practical batteries, there are mainly used in combination the high permittivity solvents such as ethylene carbonate (permittivity ε=90) and low permittivity solvents such as dimethyl carbonate (DMC, ε=3.1) and ethylmethyl carbonate (EMC, ε=2.9).
As electrolytes used in non-aqueous electrolyte batteries, there are used those which are prepared by dissolving a solute of about 1 mol in concentration in the above solvent. As the solute, there are used inorganic acid anion lithium salts such as lithium perchlorate (LiClO4), lithium borofluoride (LiBF4) and lithium phosphofluoride (LiPF6), and organic acid anion lithium salts such as lithium trifluoromethanesulfonate (LiSO3CF3) and lithium bistrifluoromethanesolfonimide ((CF3SO2)2NLi).
Furthermore, recently, completely solid polymer electrolytes or so-called gelled polymer electrolytes comprising a polymer matrix into which said electrolyte is incorporated are also often used.
Separators are those which are insoluble in the above non-aqueous electrolytes, and, for example, porous films made of polyethylene or polypropylene are used.
As to surface active agents, it has been proposed to add surface active agents to positive electrode or negative electrodes (JP-A-63-236258, JP-A-5-335018). However, when surface active agents are added to positive electrodes or negative electrodes, volume energy density or weight energy density of these electrodes lowers to cause deterioration of charge and discharge characteristics.
It has been proposed to add to non-aqueous solvents a nonionic surface active agent having an HLB (hydrophilic-lipophilic balance) of not more than 15, for example, polyoxyethylenephenyl ether, in an amount of 1×10−5-3×10−1 mol/liter (JP-A-9-161844). In this case, diffusibility of ions such as lithium ion can be improved and load characteristics of batteries can be improved without causing much decrease of energy density of the batteries. However, in these batteries, cycle life is also apt to become insufficient and amount of evolved gases is relatively large.
In addition, as for zinc alkaline batteries, a technique is disclosed in which surface active agents are added to alkaline electrolytes (aqueous solutions) to improve corrosion resistance of zinc alloys used in the batteries (JP-A-4-322060).